The ferry first sailed in May 2002
Operators of Scotland's continental ferry service have said it needs to attract more freight traffic to ensure its survival.
The warning comes on the first anniversary of the link between Rosyth in Fife and Zeebrugge in Belgium.
But Superfast Ferries said the route has been a huge success, attracting 100,000 passengers so far, many of whom were new tourists to Scotland.
The ferry service began sailing in May last year after years of campaigning for Scotland to have its own direct link to Europe.
The figure of 100,000 travellers is more than double the number expected.
The crossing to the Belgian port of Zeebrugge takes more than 17 hours.
Tourism body VisitScotland said it was bringing new visitors who tend to drive and stay for longer than those arriving by air.
Philip Riddle, chief executive of VisitScotland, said: "It's a fantastic, direct access from the continent, right into the heart of Scotland and it's taken more than 100,000 passengers already in service."
But there are concerns the service cannot survive without increased freight traffic, which is slowly growing at about 4% a month.
There are hopes freight traffic will grow
The Road Haulage Association (RHA) reports a number of major players are now using the ferry.
It also pointed to upcoming regulations restricting driver hours, which it believes could see increased freight only business.
Pat Glancey from the RHA said: "March 2005 will see the working time directive come in.
"At present the way that this ferry is working, especially with just the trailers going, it's saving on driver time, it's just the goods that are on the ferry being met at both ends."
Forth Ports, which runs the Rosyth terminal, said it was confident the link has a future.
Chief Executive Charles Hammond said the it was still early days for the route.
"We're still in a start-up period, but we're building on that and we're very confident that Superfast and the ferry service are here to stay," he said.